Compression Ratio Measurment on 2.7 RS copy
 

I am finally getting time to assemble my 2.7 RS replica motor. It is based on a 73E 2.4 7R case and I'm using Mahle RS 90mm pistons and cylinders. As youare all aware these are advertised at 8.5:1 compression ratio. I felt it might not be unreasonable to expect to bump these to 9:1 when the head mating surface was machined .010" while being bored to fit the larger cylinders. I have set the deck height to .040" (1mm).

Unfortunately, when I measure the comression ratio I come out with much lower numbers even than the 8.5:1 advertised numbers. Various posts I have read here over the years indicated that Mahle might exagerate their compression ratio numbers a bit which seems odd but I can accept that might be the case. However I'd like to hear from anyone who has seen similar results on a similar motor as I am now contemplating options such as selling the RS pistons and buying JEs.

Compression ratio was measured in two ways:

method #1:

With engine on stand rotate so that spark plug hole is vertical and high. At TDC fill combustion chamber (to bottom of spark plug threads) burp to remove last air. Piston sealed with grease. Remove ATF and head, clean. Turn to BDC re-grease install head. Refill with ATF from burette, burp and measure total volume.

Combustion chamber (at TDC) volume = 63.5ml
Total volume (at BDC) = 518ml
compression ratio = 8.15:1

method #2:

As per Bruce Anderson's book

V1=swept volume= 2687/6=447.8ml
V2=deck height volume= .7854x.1cmx9.cmx9cm= 6.36ml
V3=combustion chamber vol = 68ml
V4=piston dome volume = 10.6ml

compression ratio= (V1+V2+V3-V4)/(V2+V3-V4)

=(447.8+6.36+68-10.6)/(6.36+68-10.6)= 8.02:1


Allowing for some rounding errors and errors in measurements these two results are close enough (2%) to offer some corroberation.
Can anyone see something I am overlooking? Theses results are dishearteningly low and I am looking for opinions on a diresction to take.

Thanks for taking the time to read this.

Scott Johnston

Scott, I ran the numbers like you and agree with what you have. It looks like the only way to get 8.5:1 compression with what you have is to run a .014" deck height.
I would be tempted to talk to someone representing Mahle to understand how they come up with 8.5:1 with a very marginal deck height.
The only way the Mahle piston runs at 8.5:1 is with a dome volume near 14.6ml, with a 1mm deckheight.
The numbers don't lie. So whats the deal?

In your deck height calc:
Isn't cly volume Pi X *R* squared X H?

JP he's not using pi, if you use pi R squared h you still get 6.36ml. Not sure what the deal is with .7854 but the 6.36 number is right.

Guys;

The (Pi)xRxRxH formula that we all learned in school does indeed give the same result that one the Bruce Anderson gives in his book which uses .7854. Got me as to why?

2.7Racer;
My dome volume is no more then 10ml by direct measure. I assume that I do have RS pistons as I bought them from EBS, they are 90mm and do have a slight dome with valve cutouts.

Thanks for your input.

Scott

Scott,
I would contact EBS and discuss your problem with them. I've done a little business with them and I think everyone on this board would agree they are the "good guys".
Ultimately, I expect for you to have true 8.5:1 pistons working in a 68ml head you will need a piston with a dome height closer to the 14.6ml as calculated.
Anything less than 14.6ml dome would require you to run less than 1mm (.039") deck height.

Scott,
I believe the recommended max. cut from the cyl. head sealing surface is 0.040". Yours was cut "only" 0.010". If so, calculate the compression ratio increase. Don't forget to remove a commensurate amount from each chain box mounting surface.

Sherwood

Today I made another measurement of the various volumes using the Bruce Anderson method. I have nearly 100% confidence in the accuracy of this test and it yielded a compression ratio of 8.26:1.

2.7racer:
I will talk to EBS about this. I too have bought a lot of stuff from them over the past 10 years and always found them to give great service. My piston and cylinder box has the correct part number so I am sure they are the correct part too.

My heads probably started out with 68ml combustion chambers but I didn't measure them before having machined this time and this engine had been apart once (at least) before I tore it down so the heads may have been machined in the past.

My combustion chambers are 66ml now and with the 10.6ml dome I can see no way to achieve the desired compression ratio of around 9-9.25:1. As you note it will take a substantial increase on dome volume to get to the advertised compression ratio with a 68ml combustion chamber. I calculate that it would require about a 13ml dome to get me to 8.5:1 with my 66ml chamber.

Sherwood:
My head may have been cut in a previous rebuild but it sure looks like taking another .030" of the surface would bring me really close to the valves.

By my measurements and calculations each .010" cut on the heads will only reduce my combustion chamber volume by .36ml. Taking the full max cut of another .030" would only get me a reduction in chamber volume of 1.1ml. No where the 3ml I need to even get to the advertised 8.5:1.

I sure do appreciate the input you guys are giving. It is causing me to think more about this and spend more time on my measurements and calculations.
Thanks a lot,
Scott

You pretty much have to measure the cylinder and the head separately - doing so together may not yield the proper result. See some pictures here for details:

http://www.pelicanparts.com/swapmeet_pics/Bruce-Jerry/Page2.htm

-Wayne

Wayne;

I did use exactly the method described and used by Bruce Anderson (and shown in the photos you linked to), with the plexiglass plate covering the combustion chamber as well as the cylinder during their successive fillings with ATF.

I also used the method of filling the assembled cylinder and head more to make a comparison. This yielded a result within 2% of the BA method.

Thanks,
Scott

These are the numbers for my RS clone

V1= 447.8ml
V2= 4.14ml
V3= 66 ml
V4= 13.8 ml
Compression Ratio: 1:8.95

I am using standard Mahle P/C, heads were machined, engine case was machined and then align bored

I am very pleased with the result

When comparing Scott's numbers with Peter's I can see why peter is happy. First Peters deck height is less by just over 2ml (4.14 vs 6.36). Second by machining his heads Peter has reduced the head volume by 2ml (66 vs 68). Third Peters unmachined pistons have more than 3ml dome volume (13.8 vs 10.6).
All of these numbers work toward increasing Peter's compression ratio as compared to Scott's.
I still see a Piston dome volume difference between pistons.
As I see it Scott did NOT get 8.5:1 pistons.
Someone owes Scott an explaination.

Preterfrans:
After rechecking my measurements I found that I too have 66ml combustion chambers (V3). Of course my swept volume is the same as yours at 447.8 (V1). What was the deck height you shimed your cylinders too? Mine was 1mm which according to my calculation of;
pi(4.5cm)(4.5cm)(.1cm)=6.36cc (V2)
The biggest difference I see is that my piston dome volume measures 10.6cc (V4) as opposed to the 13.8 that your does.

Doug:
Good observations you made. Jon at EBS has gaciously offered to check the volumes in question in order to confirm my numbers. I'm sending him a piston, cylinder and head in the morning.

Will let you know what we find.
Thanks, Scott

Something does not seem right here, but I don't think Mahle made any other lower compression RS 2.7 pistons.

-Wayne

Scott,

My deckheight is 0.65mm (including the 0.25mm shim). This gave me a piston to head clearance of 1.05mm and a piston to valve clearance of 1.76mm (both measured with the solder method).

Odd that the dome volume on mine is higher than on yours. I attached a pic of the piston (its the left one).http://forums.pelicanparts.com/uploa...1127142304.jpg

Re: Compression Ratio Measurment on 2.7 RS copy
 

FYI, there is no such thing as a 73E 2.4 7R case. All the 7R cases from 1973 were from the 1973 1/2 911T CIS engines. The 911E engines used 4R and 5R case halves. In general, these are pretty valuable if they are uncut (not bored out to 2.7L). I recommend most people look for a 1974 case to build a 2.7 RS-spec engine out of...

-Wayne

Higher dome height *should* give higher compression, but you must make sure that the distance from the center of the rod to the top of the flat of the piston is the same. This distance is different for many pistons, and is the reason why installing the 2.2S pistons in a 2.4 motor increases the compression.

-Wayne

Hello Wayne.

Thats very interesting information about the 7R 2.4 cases.

Because I have a 7R case 2.4E...

Now what I'm wondering is was this a factory rebuilt motor on the later case?

Kind regards
David

Wayne;

Perhaps you are correct about there not being any E 7R cases, but mine sure seems to be.
My case has a 7R part number. It also has the 911/52 type number stamped into it. My enginew number is 6230902. As you no doubt know the type 52 is the E designation. In the engine number x2xxxxx indicates an E engine and the xx3xxxx indicates 1973 year of manufacture. All of these numbers appear original and not to have been altered in anyway.
I find it curious that my 73S has a 5R case and it has an engine number a few hundred numbers earlier. I have always assumed that Porsche changed to the 7R some time in mid 73.
Regards,
Scott

It was my understanding that the 7R cases started showing up on production cars as early as '72 (granted, very few), and from then on there was no real rhyme or reason to when they used 4R/5R or 7R cases. Thus you can have a '73 with either, though it seems all the '73.5 T's had 7R cases.

This is at least what I have gathered from reading other threads on this subject.

Brooke